This invention relates generally to electrooptical displays of the type using electrochromic material which undergoes reversible color changes upon the application of an electric field. More particularly, the invention relates to an improved sandwich construction for an electrochromic display.
It is known that some materials exhibit the property of existing in more than one oxide state wherein the material in one state exhibits different optical properties from the material in another state. Examples of such materials are set forth in U.S. Pat. No. 2,319,765--Talmey issued May 18, 1943 and U.S. Pat. No. 3,521,941--Deb et al issued July 28, 1970. It is also known that a reversible electric field applied to electrochromic materials may be used to selectively produce visible images and to erase such images as described in U.S. Pat. No. 1,068,744--Hutchison issued July 29, 1913 and in the aforementioned Deb patent.
Various systems have been used in an electrooptic display such as liquid systems with a coloring material as the electrolyte per U.S. Pat. No. 3,283,656--Jones et al issued Nov. 8, 1966 solid state systems with no electrolyte per U.S. Pat. No. 3,560,078--McIntyre et al issued Feb. 2, 1971, or systems employing a gelled electrolyte per U.S. Pat. No. 3,704,057 to Beegle issued Nov. 28, 1972.
In some cases, as with the present invention, it is preferred to employ a liquid electrolyte for ion transfer between spaced electrochromic layers. This gives rise to problems of sealing the display against leakage and providing for changes in thermal expansion of the electrolyte which can require a flexible seal. However, if very close spacing is employed, which would also reduce temperature effects and allow lower operating voltage, there is a problem of hiding the counter-electrode so as to get good contrast between the electrochromic material on the image segments and the background around the segments.
It has been suggested in U.S. Pat. No. 3,807,832--Castellion issued Apr. 30, 1974 that an electrochromic mirror, having no discrete display characters, can employ a metallic reflecting surface having pores to allow a liquid electrolyte to permeate the reflective layer. However in an image display device, it is preferably desired to have a contrasting, but not necessarily reflective, background for the optical characters of electrochromic material, dependent on the optical characteristics of the electrochromic material.
Accordingly, one object of the present invention is to provide an improved electrochromic display of the type using liquid electrolyte which provides improved resistance to the effects of temperature change.
Another object of the invention is to provide an improved electrochromic image display with improved means for providing contrasting background for the images.
Still another object of the invention is an improved electrochromic display which allows closer spacing between spaced substrates using a liquid electrolyte between electrochromic layers.